Testing ZS at a ceiling rose, not practical to undo the rose to get an earth connection, however I can toggle my fluke to L-N and get a ZS reading. Is this acceptable and if so my understanding of a ZS test is an earth fault loop impedance test! So if testing without the earth how can it be a ZS test, anybody explain why, Many thanks

I'm assuming that this is a PIR, and if so then LIM would be perfectly acceptable in the circumstances you describe.
However not being conversant with your tester myself, I suggest you look at the technical specifications or hand book.
It may be that this particular test only works on TN-C-S systems, or it may be a guesstimate, and you are required to select cable type, or input the CSA of the CPC.

If you use the L-N setting this will give you a high current test between whichever conductors you put it on. If it is going on line and neutral then it will give you the line-neutral impedance, if you put it on line-earth it will give you the EFLI (but may trip the breaker and RCD if present).

When you toggle it on the Fluke you are measuring the L-N loop not the L-E loop therefore you are not measuring the Zs or the Ze. The purpose of the toggle is to allow you to measure the prospective fault current which will be the higher reading of either the PEFC ie the Ze/Zs at that point in the installation or the PSCC ie L-N loop (short circuit)

You would use the L-N range on your Fluke if you were doing a L-N loop test or measuring PFC for instance, you can also use the L-N range for carrying out a 2 lead Zs test but in this case you would still need to test between L-E or it wouldn't be a Zs test! (it will also trip the rcd if one is in circuit) If it is a pir, surely there is some other point that's more accessible?

A Zs test is not to prove an earth connection exists, although it would do so, but to prove that the CPD would operate within the time required by BS7671.
Smpling should be of circuits, not equipment of a circuit that is being tested.

Line Impedance
Line impedance is source impedance measured between Line conductors or
Line and Neutral. This function allows the following tests:
. Line to Neutral loop impedance.
. Line to Line impedance in 3-phase systems.
165XB
Users Manual
22
. L-PE loop measurement. This is a way of making a high current, 2-wire
loop measurement. It cannot be used on circuits protected by RCDs
because it will cause them to trip.
. Prospective Short Circuit Current (PSC). PSC is the current that can
potentially flow if the phase conductor is shorted to the neutral conductor or
another phase conductor. The tester calculates the PSC current by dividing
the measured mains voltage by the line impedance.

To measure line impedance:
1. Turn the rotary switch to the x position. The LCD indicates that the
high current loop mode is selected by displaying the o symbol.
2. Connect the red lead to the L (red) and the blue lead to the N (blue)
terminals of the tester.
3. Press 1 to select L-N.
4. Use the zero adapter to zero the test leads or the mains cord.
5. Press and hold Z for more than two seconds until the ZERO
annunciator appears.

Testing ZS at a ceiling rose, not practical to undo the rose to get an earth connection, however I can toggle my fluke to L-N and get a ZS reading. Is this acceptable and if so my understanding of a ZS test is an earth fault loop impedance test! So if testing without the earth how can it be a ZS test, anybody explain why, Many thanks

This is not an Earth Loop Impedance then is it, This will be a line to neutral impedance you will be measuring. The fluke certainly does have this option but also is used to measure PSC at the same time.

You have done it now. I raised te issue of calculating Zs on the forum sometime ago after watching the NICEIC Domestic PIR DVD. On the DVD Tony Cable derives the value of Zs from Ze + R1 +R2 pointing out compliance with Regulation 14. Having looked at I agree you should not live test Zs if you can avoid to comply with Reg.14.

Well the old school on the forum thought this was an assault on their experience, knowledge and man hood. The tuskers came roaring out of the long grass and I had to retreat.